A semiconductor laser device is an optical device necessary for an optical disc apparatus to write data onto optical discs, such as DVDs (Digital Versatile Disc) and CDs (Compact Disc).
In recent years, the optical disc apparatus has been required to increase the writing speed. In order to increase in speed, semiconductor lasers need to have a higher output.
FIG. 9 is a cross-sectional view showing the structure of a conventional high-power semiconductor laser device 20 disclosed in Patent Document 1.
The semiconductor laser device 20 is formed on an n-type GaAs substrate 10. On the substrate 10, an n-type GaAs buffer layer 11, an n-type AlGaInP first clad layer 12, an AlGaInP active layer 13, a p-type AlGaInP second clad layer 14, a p-type GaInP protective layer 15 are laminated in sequence.
In this way, the semiconductor laser device 20 has a double-hetero structure in which the active layer 13 is sandwiched between the clad layers 12 and 14 that each have a larger energy band gap than the active layer 13.
Also, as shown in FIG. 9, the p-type AlGaInP second clad layer 14 forms, on the active layer 13, a protruding ridge whose upper surface is flat. Furthermore, the ridge is partly covered with an n-type AlGaInP current block layer 16 in a manner that the upper surface of the protrusion is exposed. Also, a p-type GaAs contact layer 17 is laminated so as to cover the n-type AlGaInP current block layer 16 and the p-type GaInP protective layer 15 that is positioned on the upper surface of the protrusion.
To improve the light extraction efficiency of the high-output semiconductor laser device 20, a front cleavage plane of a resonator is coated such that the reflectivity thereof is approximately in a range of 5 to 10%, and a rear cleavage plane of the resonator is coated such that the reflectivity thereof is approximately in a range of 95 to 100%.
Therefore, in terms of the spectral distribution intensity generated in the semiconductor laser device 20, the front cleavage plane and the rear cleavage plane are asymmetric. In other words, the spectral distribution intensity in the vicinity of the front cleavage plane is approximately twice as high as the spectral distribution intensity in the vicinity of the rear cleavage plane. As a result, a region in the vicinity of the front cleavage plane, whose optical density is higher, needs more light amplified by stimulated emission (hereinafter referred to as “stimulated emission light”) than a region in the vicinity of the rear cleavage plane, whose optical density is lower. Therefore, the active layer 13 in the region in the vicinity of the front cleavage plane needs more pairs of electron and positive hole (hereinafter the pairs are referred to as “carriers”) than the active layer 13 in the region in the vicinity of the rear cleavage plane, so as to cause more stimulated emission light to be generated.
Therefore, in the semiconductor laser device 20, the width (equivalent to the width of the p-type GaInP protective layer 15) of the upper surface of the ridge (protrusion) is formed so as to be smaller linearly from the front cleavage plane to the rear cleavage plane, in a direction of the resonator where the ridge extends (in an orthogonal direction with respect to the paper surface of FIG. 9), as shown in FIG. 10.
Specifically, the upper surface of the ridge, in a region from the front cleavage plane to the rear cleavage plane, is in the shape of an inverted trapezoid.
The above-described structure makes it possible to inject a greater number of carriers into the active layer 13 in the region in the vicinity of the front cleavage plane, and that has a higher optical density. Therefore, gain saturation is not easily caused even at a high output power, thereby achieving a high thermal saturation level. At the same time, a portion where the width of the upper surface of the ridge is narrow suppresses a high order traverse mode oscillation, resulting in preventing a kink from occurring.
Patent Document 1: Japanese Laid-Open Patent Application No. 2005-209952